(Note: Developer, small dev shop, higher-priced software, same situation.)
If you distribute an "unlimited" version, this will be what is pirated; there's no value in having different versions. Also, if you have a key which allows "unlimited" access without secondary verification, this is what will be distributed on pirate sites.
In our experience, it took about a week from changing the key format to a new crack key being distributed. Obviously, this is for software which is "in-demand", but don't expect that implementing a new scheme with the same underlying characteristics will buy you much time.
For "good" protection, you basically need secondary verification which is "hard" to fake. Currently, that is hardware dongles or an online verification loop. Both of these can be pains for the users, costly for you, and/or prohibitive in some environments (online, in particular, doesn't play nice with classified government envs).
Keep in mind also: most people who pirate are not potential customers, at least at anything close to full price, but their experience using the tool may turn into a sale at a company later.
My suggestion: do what you can to track usage, but don't be overly obtrusive and/or try to prevent all piracy usage. Being able to watch and track, and act when appropriate, is much better than trying to prevent all piracy.